Conventionally, an antenna device (also known as a vehicle roof mount antenna device) mounted on a surface of a vehicle roof is provided.
Generally, a surface of a vehicle roof is curved to have a smoothly projected shape. Thus, as shown in patent literature 1, when an antenna device is mounted on a surface of a vehicle roof at a back portion of the roof surface, a projected portion of the roof surface positioned in a front direction of the antenna device obstructs an operation of the antenna device. As a result, a gain of the antenna device in a front direction of the vehicle is difficult to be secured. Particularly, high-frequency radio wave having operating frequency band of gigahertz (GHz) band has a high rectilinearity and readily attenuates with an increase of a transmission distance. Thus, in the antenna device receiving the high-frequency radio waves having GHz frequency band, securing the gain in the front direction of the vehicle is especially important.